The present invention relates generally to an ultrasonic distance sensor suitable for precise measurement of the distance to an object. More specifically, the invention relates to a distance sensor transmitting ultrasonic waves toward an object, receiving reflected ultrasonic waves and deriving the distance to the object based on an elapsed time between transmission of the ultrasonic waves and reception thereof. Further, the invention relates to application of the ultrasonic distance sensor to monitoring or scanning the surface profile of an object for detecting the external shape of the object. Such monitoring of the external shape of the object may be used to monitor the profile of a rolled product during rolling, for example.
In the steel industry and especially in rolling steel, ultrasonic distance sensors have been used for precise distance measurement or surface monitoring. In the known art, the ultrasonic waves are transmitted through a water tank or a water jet to an target surface to measure the distance by measuring the elapsed time between transmission and reception of the reflected ultrasonic waves. However, during transmission, water temperature variations tend to deviate the transmission time which causes errors in the measured distance to the target surface.
In order to eliminate the influence of the water temperature, it is necessary to correct the measured elapsed time or derived distance. This water temperature-dependent correction of the measured value was proposed in "Materials Evaluation" Vol. 35, No. 2, published in 1977, on pages 45 to 50. In the proposed system, an auxiliary sensor monitors the elapsed time to reception of an ultrasonic wave reflected by a reference surface which is located at a known distance from the auxiliary sensor. Based on the elapsed time measured by the auxiliary sensor, the transmission velocity of ultrasonic waves through the water is derived. The elapsed time measurement then utilizes the transmission velocity of the ultrasonic wave derived with respect to the reference surface. This proposed system helps improve the accuracy of the resultant measured distance.
However, the prior proposed system set forth above requires primary and auxiliary sensors for water temperature dependent correction, which increases the overall cost. Also, since the points of measurement of the primary and auxiliary sensors are separate, their water temperatures will tend to differ, so that the correction value will not necessarily be accurate. As is well known, the variation of transmission velocity of the ultrasonic waves relative to the water temperature is about 1.6.times.10.sup.-3 /.degree. C. This means when a temperature difference of 0.1.degree. C. exists between the measurement points and the distance to be measured is about 30 mm., the error of the distance measurement will be about 5 .mu.m. This error is unacceptably large for precise measurement of the distance.
Furthermore, the aforementioned known system utilizes a ramp wave and samples the ramp wave voltage upon reception of the reflected ultrasonic waves to measure the elapsed time. Accuracy of this system is limited to the linearity of the ramp wave and thus is rather low, at an error of about 0.1%. This means when the distance being measured is 30 mm., the error will be about 30 .mu.m. The error due to the temperature difference between the two measuring points will be superimposed on the fundamental error set forth above.
In addition, various modern industries require highly accurate products, such as rollers for the rolling process and so forth. The foregoing conventional system is not able to satisfy the requirements for accuracy.